.alpha.-cyano-3-phenoxy benzyl alcohol compounds have been known as the novel alcoholic moiety of a series of synthetic pyrethroid ester compounds which exhibit excellent insecticidal activity. Synthetic pyrethroid esters such as, for example, fenvalerate, cypermethrin, deltamethrin, have an .alpha.-cyano-3-phenoxy benzyl alcohol moiety and are good insecticides for household and sanitary uses as well as agricultural use, since such synthetic pyrethroid esters possess sufficient photostability combined with significant insecticidal activity.
Since the .alpha.-cyano-benzyl alcohol compound has an asymmetric carbon at the .alpha.-position thereof, there are two optical isomers. With respect to insecticidal activity as an ester, the (S)-isomer ester of the .alpha.-cyano-benzyl alcohol compound is much superior to the antipode (R)-isomer ester. (See Hirosuke Yoshioka, Fournal of Synthetic Organic Chemistry, Japan, vol. 38, No. 12, pp. 1151-1162 (1980)). Accordingly, the development of commercially useful technology has been desired in order to optically resolve the (R,S)-isomer of said .alpha.-cyano-benzyl alcohol compound to obtain the (S)-isomer thereof and particularly the resolved (or at least substantially resolved) (S)-.alpha.-cyano-3-phenoxy benzyl alcohol.
However, since the .alpha.-cyano-benzyl alcohol compound is unstable, the optical resolution thereof is not easy. Methods now available for the optical resolution of this alcohol are complicated and require expensive optically active agents.
The conventional optical resolution process for alcohols is effected via an ester or a half-ester with an ordinary optically active organic acid may not be advantageous for use in resolving the (R,S)-isomer of the .alpha.-cyano-3-phenoxy benzyl alcohol compound. Methods for obtaining (S)-.alpha.-cyano-3-phenoxybenzyl alcohol include the following: (1) a method wherein the desired (S)-.alpha.-cyano-3-phenoxybenzyl alcohol is obtained by reacting (R,S)-.alpha.-cyano-3-phenoxybenzyl alcohol with cis-2,2-dimethyl-3(S)-(dihydroxy-methyl)-cyclopropane-1(R)carboxylic acid lactone in the presence of an acidic reagent to obtain an ether compound, separating each isomer from the resulting mixture of two kinds of the isomers using physical separation techniques, and hydrolyzing the ether compound containing (S)-isomer alcohol component in an acidic medium (see Japanese Unexamined Published Patent Application No. 109944/1979); and (2) a method wherein (S)-.alpha.-cyano-3-phenoxybenzyl alcohol is obtained by reacting an (S)-.alpha.-cyano-3-phenoxybenzyl alcohol ester of a chiral cyclopropane-carboxylic acid with a boron halide and then with water (see Japanese Unexamined Published Patent Application No. 12355/1981). However, these methods are complicated and require an expensive optically active reagent. The overall yields are also not particularly high. Furthermore, in method (2), the optically active .alpha.-cyano-3-phenoxybenzyl alcohol ester has to be prepared in advance. Based on these points, the methods heretofore known can not necessarily be said to be satisfactory.